1. Field of the Invention
The present invention relates to surface wave acoustic devices and, more particularly, to a novel method and apparatus for performing correlation and related signal convolution functions through the use of combined surface wave acoustic techniques and semiconductor techniques.
2. State of the Prior Art
Signal correlation and related signal convolution functions as applied to various types of signals typically involve the detection of one or more particular characteristics of a signal or the combining of signals in such a way that a particular characteristic is reinforced or suppressed. For example, signal correlation may involve the examination of a signal for a predetermined code carried by the signal in the form of pulse spacing or repetition frequency and the generation of an appropriate response when the proper code is detected. This type of signal correlation may be accomplished through the comparison on two signals (e.g., cross correlation) or through self or auto correlation techniques involving the use of frequency sensitive filters or the like.
One known type of self or auto correlator referred to as a Barker code decoder employs surface wave acoustical devices to provide a filter which is matched precisely with the characteristics of the desired signal and rejects undesired signals. To decode a signal in this manner, a surface wave acoustic device having a pickup or output transducer appropriately matched to the characteristics of the desired signal is typically employed in conjunction with a substrate of acoustic material such as quartz. The result is that only these surface waves having the desired characteristics are detected and undesired signals are rejected.
The foregoing type of correlators may be utilized successfully in connection with the decoding of signals with unvarying characteristics. However, since the correlation function depends upon fixed filtering characteristics of the correlator, neither the correlating function nor the correlated signal may be altered. Cross and auto/cross correlation between two signals, in effect, is thus typically required where variations in the correlating characteristics are expected and/or desired.
Cross and auto/cross correlation, and related convolution functions require essentially three functions, that is, (1) storage of the two signals over the integration period in real time, (2) real time multiplication of the two signals and, (3) integration. An acoustic medium such as the substrate of a surface wave acoustic device can be used for one or more of these functions. Clearly, the acoustic medium is quite suitable for the storage function. Attempts have been made to use the acoustic medium for the multiplication function as well by taking advantage of the nonlinearity of the acoustic medium for correlation of traveling wave harmonic generation or conversion of closely related functions.
From an analysis of conversion efficiencies for correlation and traveling wave operations using an acoustic medium to perform the nonlinear multiplication function, it appears that the conversion loss for correlation is much greater than the conversion loss for traveling wave harmonic generation or for frequency conversion. The reason for the difference is that correlation must occur over a relatively short interaction distance equal to approximately the wavelength of the reciprocal bandwidth of the correlating signal whereas traveling wave harmonic generation and conversion have significant distances in which to build up. It is clear that the nonlinear properties of the acoustic medium can be utilized provided the interaction time is sufficiently great. However, for correlation of broadband signals this is impossible since the interaction time is essentially the reciprocal bandwidth.